1. Field of the Invention
The present invention relates to methods and apparatus for burning fuel-air mixtures whereby flue gases having low NO.sub.x content are produced.
2. Description of the Prior Art
As a result of the adoption of stringent environmental emission standards by government authorities and agencies, methods and apparatus to suppress the formation of oxides of nitrogen (NO.sub.x) in flue gases produced by the combustion of fuel-air mixtures have been developed and used heretofore. For example, methods and apparatus wherein fuel is burned in less than a stoichiometric concentration of oxygen to intentionally produce a reducing environment to CO and H.sub.2 have been proposed. This concept has been utilized in staged air burner apparatus wherein the fuel is burned in a deficiency of air in a first zone producing a reducing environment that suppresses NO.sub.x formation, and then the remaining portion of air is introduced into a second zone. Methods and apparatus have also developed wherein all of the air and some of the fuel is burned in a first zone with the remaining fuel being introduced into a second zone. In this staged fuel approach, an excess of air in the first zone acts as a dilutent which lowers the temperature of the burning gases and thereby reduces the formation of NO.sub.x. Other methods and apparatus have been developed wherein flue gases are combined with fuel-air mixtures to dilute the mixtures and thereby lower their combustion temperatures and formation of NO.sub.x.
While the prior art methods and burner apparatus for producing flue gases having low NO.sub.x content have achieved varying degrees of success, there still remains a need for improvement in such methods and burner apparatus whereby low NO.sub.x content flue gases are produced and simple economical burner apparatus is utilized.